1. Field of the Invention
The present invention relates to a nuclear fuel rod for fast reactors, which includes an oxide coating layer formed on the inner surface of cladding.
2. Description of the Prior Art
Nuclear fuel in fast reactors is designed in various types, including a metallic type, an oxide type, a nitride type and a carbide type, and the nuclear fuel rod includes fissionable fuel slug that undergoes a nuclear reaction. The fissionable fuel slug is sealed by a cladding tube, which is not reactive due to its good compatibility with coolant (e.g. liquid sodium in Sodium-cooled Fast Reactor) and has good heat transfer properties. The fuel rods are assembled at constant intervals to construct a fuel assembly and the assemblies are put into a nuclear reactor core. Herein, the cladding surrounding the nuclear fuel slug should serve to prevent direct contact between the nuclear fuel slug and the coolant, such that chemical interactions therebetween do not occur, and to prevent the leakage of fission products into the coolant. In addition, in fast reactors, in which metallic nuclear fuel is used, it is highly advantageous in terms of the safety and economic efficiency of nuclear fuel to suppress interactions between the cladding tube and the fissionable fuel slug.
Particularly, in fast reactors, in which metallic fuel is used, there occurs a phenomenon in which the constitutional elements (U, Pu, Th, MA, Zr, Mo, fission products, etc.) of a fissionable fuel slug and the constitutional elements (Fe, Cr, W, Mo, V, Nb, etc.) of stainless steel cladding tube can interact with each other by diffusion, leading to a decrease in the melting temperature of the metallic fuel slug and a decrease in the strength of the cladding. For this reason, the maximum permissible burnup and maximum permissible operating temperature can be limited [J. Nucl. Mater., 204 (1993) p. 244-251 and J. Nucl. Mater., 204 (1993) p. 141-147].
Also, a diffusion couple experiment, carried out by T. Ogata et al. at 923 K, demonstrated that the reaction between the metallic fuel slug and the cladding tube occurred due to mutual diffusion therebetween, and reported that the thickness of a layer resulting from the mutual reaction increased with an increase of the reaction time [J. Nucl. Mater., 250 (1997) p. 171-175].
In an attempt to prevent the mutual diffusion, General Electric (GE) suggested a technology for suppressing the mutual interaction between the metallic fuel slug and the cladding tube by inserting a liner or a sleeve about 50-μm thick between the metallic fuel slug and the cladding tube using one metal selected from among zirconium (Zr), titanium (Ti), niobium (Nb) and molybdenum (Mo).
The technology of GE has difficulties in that, because the introduction of additional processes is necessary, the production of the nuclear fuel rod becomes complicated, and in that significant additional costs are incurred.
Also, D. C. Crawford et al melted and cast a zirconium tube about 200 μm thick in order to remove quartz tube mold waste occurring during the fuel slug manufacturing process for fast reactors and, at the same time, to suppress fuel-cladding chemical interactions (FCCI) between the metallic fuel slug and the cladding tube, and reported the results of experiments carried out using the zirconium tube as a inner liner. However, there was a problem in that cracks occur in the zirconium tube.
Accordingly, the present inventors have invented a nuclear fuel rod for fast reactors, in which an oxide coating layer is formed on the inner surface of the stainless steel cladding tube in order to prevent or reduce the diffusion and interaction of elements between the stainless steel cladding tube and the nuclear fuel slug, and which is easily manufactured using the prior methods for manufacturing oxides.